A semiconductor device in which an electrode portion of a semiconductor element and a conductive portion of a substrate are bonded together is used in an extraordinarily wide range. For bonding the electrode portion of a semiconductor element and the conductive portion of a substrate, a conductive adhesive or soldering is generally used. The conductive adhesive has the advantage that bonding can be performed at a lower temperature than soldering, but has the drawback that the bulk resistance is higher than soldering. For this reason, studies are conducted for reducing the resistance of the conductive adhesive.
A typical conductive adhesive includes silver as a conductive filler. However, silver has migration properties and has increased in price. Therefore, use of copper as a conductive filler is under consideration. In addition, for simplifying the manufacturing process and reducing the cost and the like, it is also required that copper, which is susceptible to oxidation, of this copper-containing conductive adhesive be cured in an ambient atmosphere.
As a paste which contains copper as a conductive filler, there is disclosed a conductive copper paste which includes as essential components a copper powder having a predetermined particle size distribution and tap density, a thermosetting resin, an organic carboxylic acid, and a chelating agent, as well as polybutadiene (claim 1 and paragraphs 0013 and 0022 of PATENT LITERATURE 1).
This conductive copper paste is intended to be suitable for through holes corresponding to fine pitch by having a screen printing capability, favorable conductivity comparable to a conductive silver paste, and migration resistance properties in combination (paragraph 0008 of PATENT LITERATURE 1). It is disclosed that specific examples of the organic carboxylic acid may include salicylic acid, benzoic acid, tartaric acid, citric acid, maleic acid, succinic acid, fumaric acid, malonic acid, and the like (paragraph 0018 of PATENT LITERATURE 1). It is noted that all of these organic carboxylic acids are solid at normal temperature.
Another disclosed example is a conductive paste for circuit boards which contains a copper-containing metal powder, a compound having at least two (meth) acryl groups, and a β-dicarbonyl compound and which does not substantially contain an azo compound or a peroxide (claim 1 of PATENT LITERATURE 2). It is described that this conductive paste for circuit boards may contain a compound having flux activity (paragraph 0014 of PATENT LITERATURE 2). It is disclosed that an example of the compound having flux activity may include an aliphatic carboxylic acid such as oleic acid (paragraphs 0038 and 0046 of PATENT LITERATURE 2).
Further another disclosed example includes a conductive copper paste composition which contains a prepolymer having at least two hydroxyl groups and at least one tertiary amine in one molecule, a copper powder, an amino resin, and a reducing agent, and which can be etched with an acidic etchant (claim 1 of PATENT LITERATURE 3). It is disclosed that an example of the reducing agent may include an unsaturated monocarboxylic acid having 12 to 23 carbon atoms such as oleic acid and linoleic acid (paragraph 0016 of PATENT LITERATURE 3).
However, it has become clear that these conductive copper pastes have the problem that the specific resistance becomes high when the curing condition is high temperature and short time (for example, at 210° C. for 10 minutes).